Ichthyosaurus marine dinosaur, computer illustration. These marine reptiles lived during the late Triassic and early Jurassic periods, around 240 million years ago until becoming extinct, along with the dinosaurs, around 65 million years ago. The ichthyosaurs resembled dolphins, having streamlined bodies, a long snout, powerful tail and large eyes. They probably ate fish, living entirely in the water and giving birth to live young.